1. Field of the Invention
The present invention relates to a superconducting magnet device having a coil (hereinafter, referred to as a disturbance magnetic field compensation coil) that compensates for a disturbance magnetic field having adverse influences on imaging by means of MRI in a medical tomographic imaging apparatus (hereinafter, referred to as an MRI apparatus) employing a magnetic resonance phenomenon occurring in a static magnetic field source (hereinafter, referred to as a main coil) that generates a static magnetic field using a superconducting coil.
2. Background Art
Generally, magnets generating a static magnetic field needed for an MRI apparatus include a permanent magnet, a normal conducting magnet, a superconducting magnet, and so forth, and a superconducting magnet is currently the mainstream owing to the size of a static magnetic field and temporal stability. In the static magnetic field source using the superconducting magnet, the generated magnetic field is so strong that chiefly two methods are adopted to prevent leakage of the magnetic field to the outside. Magnets are largely classified into two types according to the methods. One type is used in a method (passive shielding method) of covering the magnet main body with an iron body and the other type is used in a method (active shielding method) of disposing a superconducting coil of the reverse polarity instead of the iron body. Of these two types, the mainstream is the magnet used in the active shielding method because of lightness and compactness of the main body.
Meanwhile, the MRI apparatus is installed in various places and environments. The MRI apparatus may be installed in a place neighboring a road in one case and it may be installed in a place in close proximity to a power cable for an electric train or for power transmission in another case. In such cases, a variable magnetic field (hereinafter, referred to as the disturbance magnetic field) of a non-negligible size is flowing into an imaging space from outside during imaging by means of MRI because of a large iron body that is approaching or influences of an alternating magnetic field. In the case of the superconducting magnet used in the passive shielding method, the disturbance magnetic field seldom raises a problem because the iron body has the self-shielding effect. However, in the case of the superconducting magnet used in the active shielding method, the disturbance magnetic field flows into the imaging space almost intact unless some measure is taken and is therefore highly likely to have significant adverse influences on imaging by means of MRI.
In order to suppress (compensate for) the influences of the disturbance magnetic field, a superconducting coil exclusively used to compensate for a disturbance magnetic field (disturbance magnetic field compensation coil) is disposed besides the main coil for the following reason. That is, because a current is induced into the disturbance magnetic field compensation coil in a case where a disturbance magnetic field flows inside and a compensation magnetic field is generated, by cancelling out the disturbance magnetic field with the compensation by the disturbance magnetic field compensation coil and the compensation by the main coil (albeit relatively minimal), it becomes possible to suppress a variation of the magnetic field in the imaging space to several % or less of a quantity of the disturbance magnetic field that has flown inside.
The main coil is a superconducting coil and a large current normally flows in a permanent current mode. However, should a superconducting state be broken (hereinafter, referred to as quench) for some reason, it releases large energy at a time. Although most of the energy is released in the form of heat, in a case where the main coil and the disturbance magnetic field compensation coil are magnetically coupled to each other, the energy is released to the disturbance magnetic field compensation coil in the form of electromagnetic induction.
In this instance, although it depends on the degree of magnetic coupling, a relatively large current is induced into the disturbance magnetic field compensation coil and the magnetic field generated by the main coil has not attenuated sufficiently in most cases. Consequently, an extremely large electromagnetic force is applied to the disturbance magnetic field compensation coil itself. This is attributed to the fact that the disturbance magnetic field compensation coil has fewer turns than the main coil for the reasons of the cost and the installation space and a current as large as or larger than the current (for example, 400 to 700 A) in the main coil is induced into the disturbance magnetic field compensation coil in some cases. Nevertheless, because the disturbance magnetic field compensation coil has fewer turns and hence a smaller volume, it is difficult to provide the disturbance magnetic field compensation coil with sufficient strength.
It is therefore necessary to take a measure not to induce a large current into the disturbance magnetic field compensation coil as less frequently as possible, for example, by inhibiting a current of several tens Amperes or more from flowing. However, because the disturbance magnetic field compensation coil is normally formed of a superconducting coil, a current of several tens Amperes readily flows even when a considerably poor superconducting wire material is used. Herein, an iron body that has approached the disturbance magnetic field may possibly halt and in a case where the disturbance magnetic field compensation coil is formed of a copper wire or the like, an induced current will be attenuated shortly, which makes compensation over a long period impossible. This is the reason why the disturbance magnetic field compensation coil is formed of a superconducting coil. That is to say, the disturbance magnetic field compensation coil is formed by taking such an attenuation time constant into account.
It thus becomes crucial to reduce the magnetic coupling between the main coil and the disturbance magnetic coil compensation coil as small as possible preferably in the design stage. Because high homogeneity of the static magnetic field is required for an MRI superconducting magnet, the main coil is often formed by connecting a plurality of coils in series. In reference to this configuration, the disturbance magnetic field compensation coil is formed of the same number of coils, which are also generally connected in series. It should be noted that the main coil and the disturbance magnetic field compensation coil form individual closed circuits that are independent from each other.
The magnetic coupling between the main coil and the disturbance magnetic field compensation coil may be reduced by spacing apart these coils. However, for the reasons of space and structural members, the magnetic coupling is reduced basically by superposing the respective disturbance magnetic field compensation coils on the corresponding main coils while being electrically insulated from each other and by appropriately selecting the numbers of turns (turn number ratio) of the main coils and the disturbance magnetic field compensation coils (see Japanese Patent No. 3043478).
An example is set forth in Table 1 below. In this example, each of the main coil and the disturbance magnetic field compensation coil has six pairs of coils and all the disturbance magnetic field compensation coils are superposed on the corresponding main coils (detailed positions and dimensions are omitted).
TABLE 1Example of the numbers of turns of the main coiland the disturbance magnetic field compensation coil in therelated artNo. 1No. 2No. 3No. 4No. 5No. 6Main coil247546969−4202700−1376Disturbance2580805090515magnetic fieldcompensation coil(“−” indicates turns in the reversed direction and the disturbance magnetic field compensation coil has a total of 840 × 2 turns)
In this instance, a variation of the magnetic field in the imaging space is found to be about 4.2% of a quantity of the disturbance magnetic field that has flown inside when logically computed from the relative turn number ratios of the disturbance magnetic field compensation coils No. 1 through No. 6 and therefore satisfies a general target value, that is, 5% or less.
The self-inductances and the mutual inductance of the main coil and the disturbance magnetic field compensation coil in this instance are as follows:
Self-inductance of the main coil: 37.790 H (henries)
Self-inductance of the disturbance magnetic field compensation coil: 3.095 H (henries)
Mutual inductance of the both coils: 0.014 H (henry)
A change of current flowing in each coil after the quench of the main coil is shown in FIG. 7. As has been described, because the mutual inductance of the both coils is extremely small, for example, in a case where the main coil in which a current of 500 A has been flowing in a steady state is quenched and the current is reduced to 0 A, only a current of about 2.3 A is induced into the disturbance magnetic field compensation coil without having to take consumption by heat into account. Accordingly, a problem resulting from the electromagnetic force generated by the induced current as described above hardily occurs.
The main configuration of a superconducting magnet in the related art will be described with reference to FIG. 8. Referring to the drawing, the superconducting magnet 100 is formed of a main coil 200 that generates a static magnetic field in the imaging space of an MRI apparatus and a disturbance magnetic field compensation coil 310 that suppresses (compensates for) influences of a disturbance magnetic field flowing into the imaging space, and each coil is formed of a superconducting coil. The main coil 200 and the disturbance magnetic field compensation coil 310 form individual closed loop circuits that are electrically independent from each other. Basically, respective coils forming the disturbance magnetic field compensation coil 310 are disposed on the corresponding coils forming the main coil 200. In the drawing, signs, “+” and “−”, in the respective coils forming the main coil 200 and the disturbance magnetic field compensation coil 310 indicate the winding directions of the respective coils.
The main coil 200 is divided to two circuits with the use of diodes 221: one is a coil group 210 (coils 201a, 201b, 202a, 202b, 203a, 203b, 204a, and 204b) having relatively small energy and the other is a coil group 211 (coils 205a, 205b, 206a, and 206b) having relatively large energy. Owing to this configuration, a risk of consuming large energy by a coil having a small volume (heat capacity) is lowered and protection against generated voltage and energy consumption (heat generation in the coil) at the time of quench is provided.
However, regardless of which coil is quenched, a noticeable difference of currents is unavoidably generated between the two diode circuits. This difference of currents varies with a coil that is quenched, a quench back method, and so forth. An example of an image of currents in the both coils after the quench of the main coil is shown in FIG. 9. In this example, coils No. 1 through No. 4 in Table 1 above are given as a coil group having small energy and coils No. 5 and No. 6 are given as a coil group having large energy. In this case, as is obvious from the drawing that a difference of currents between the group having large energy and the group having small energy is as large as about 300 A at the maximum. Accordingly, a large current is also induced transiently into the disturbance magnetic field compensation coil, which possibly reaches as large as about 200 A at the maximum.
In other words, it is understood that no matter how small the magnetic coupling (mutual inductance) between the main coil and the disturbance magnetic field compensation coil is made, a large current is induced temporarily into the disturbance magnetic field compensation coil at the time of quench when the magnetic coupling (mutual inductances) between each of the coil group having small energy and the coil group having large energy in the main coil and the disturbance magnetic field compensation coil is large. In this example, the mutual inductances between the respective coil groups in the main coil and the disturbance magnetic field compensation coil are as follows:
Mutual inductance between the group having small energy in the main coil and the disturbance magnetic field compensation coil: 2.206 H (henries)
Mutual inductance between the group having large energy in the main coil and the disturbance magnetic field compensation coil: −2.192 H (henries)
Mutual inductance between the main coil (whole) and the disturbance magnetic field compensation coil: 0.014 H (henry)
It is understood from this result that the mutual inductances between the respective groups in the main coil and the disturbance magnetic field compensation coil are about 160 times larger than the mutual inductance between the main coil and the disturbance magnetic field compensation coil. Accordingly, a current of about 200 A at the maximum is induced into the disturbance magnetic field compensation coil.
In this instance, a large electromagnetic force is generated in the disturbance magnetic field compensation coil and a concrete example is set forth in Table 2 below.
TABLE 2Maximum electromagnetic force applied to thedisturbance magnetic field compensation coil in example ofquench in the related art(Unit: ton)No. 1No. 2No. 3No. 4No. 5No. 6Electromagnetic force−0.220.491.833.510.305.10in Z directionElectromagnetic force−0.70−2.17−3.622.68−20.7080.18in R direction
It is understood from Table 2 above that an electromagnetic force in the R direction of the coil No. 6 is particularly large. In many cases, a space on the periphery of the coil No. 6 is tight for the reason of installation and it is therefore difficult to take a measure against the electromagnetic force in the R direction. Hence, the most urgent and crucial issue is to reduce this electromagnetic force. From Table 1 above, the disturbance magnetic field compensation coil No. 6 has 515 turns and it is another issue to reduce the number of turns for the reason of space. However, in the design of a normal disturbance magnetic field compensation coil, when the magnetic coupling with the main coil is reduced, the number of turns of the coil No. 6 has to take a relatively large value. It is therefore difficult to reduce the number of turns.
Meanwhile, in view of the foregoing, it is desirable to reduce the magnetic coupling (mutual inductances) between each of the coil group having small energy and the coil group having large energy in the main coil and the disturbance magnetic field compensation coil. It is, however, desirable to reduce the mutual conductance of the main coil (whole) and the disturbance magnetic field compensation coil at the same time. The reason why is as follows. That is, it is necessary for the disturbance magnetic field compensation coil to take into account not only a current induced when the main coil is quenched but also a current induced in a case where the main coil is magnetized or demagnetized or in a case where a superconducting portion in a persistent current switch (PCS) 220 (see FIG. 8) provided inside the superconducting magnet is quenched, and in such a case, the induced current is determined by the mutual inductance of the main coil (whole) and the disturbance magnetic field compensation coil.
As has been described, in a superconducting magnet device provided with a disturbance magnetic field compensation coil to suppress a variable magnetic field flowing into the MRI apparatus from outside, for example, even when the mutual inductance between the whole main coil and the disturbance magnetic field compensation coil is minimized in order to minimize an induced current generated when the main coil is quenched, there is a difference of currents between the respective diode circuits at the time of quench in a case where the main coil has a plurality of protection diode circuits. The superconducting magnet device therefore has a problem that a large induced current is generated by the mutual inductances between the respective diode circuits and the disturbance magnetic field compensation coil.